A conventional start circuit 200 for a forward/reverse rotating fan is illustrated in FIG. 3. The start circuit 200 drives a stator coil 300 of a motor to make the rotation direction of the fan switched from one direction into another. The start circuit 200 comprises a Hall IC 210, a driver IC 220 electrically connected to the Hall IC 210 and an activation unit 230 connected to the Hall IC 210, wherein the activation unit 230 comprises a first activation circuit 231 and a second activation circuit 232. The first activation circuit 231 and the second activation circuit 232 are driven simultaneously so as to change the polarity of the signal received by the Hall IC 210. Meantime, the stator coil 300 is driven by the driver IC 220 to switch the rotation direction of the fan. However, when the rotation direction of the fan is changed, the stator coil 300 likely generates spike phenomenon to influence the life time of the motor and damage the driver IC 220 and relative devices.